Generally, cell sorting devices separate cell populations of interest from a suspension and/or other types of cells. The principal method of operation of early cell sorting devices relied on a cell's physical parameters to distinguish that cell from a suspension and/or other types of cells. Examples of bulk cell sorting techniques include filtration, which is based on cell size, and centrifugation, which is based on cell density. These techniques are effective as long as the cell population of interest is significantly different, with respect to size or density, from the suspension and/or the other cells in the population (i.e. a separation of red blood cells from blood.) However, when the cell population of interest does not differ significantly in size or density, the filtration and centrifugation techniques are ineffectual.
In attempting to address this problem, a technique was developed which did not rely on cell size or density differences relative to the suspension and/or the other cells in the population. This technique was based on the presence of a tagging element which was attached to the surface of the cell. This tagging technique has evolved to become a significant analytical tool in basic biological studies, applied biological studies, in the clinical diagnosis of diseases and the rapidly developing cell-based therapies in the treatment of diseases.
One application of this tagging technique is known as Fluorescence-Activated Cell Sorting (hereinafter FACS). In the FACS technique, an antibody-fluorescent label conjugate is used to tag a specific cell surface marker. The primary mode of operation of a FACS sorter is binary in nature, that is, it determines whether a cell has the threshold number of fluorescent labels (i.e. positive sorting) or it does not have the threshold number of fluorescent labels (i.e. negative sorting). This determination is made by passing cells, single file, through a device which can determine whether each cell includes the parameter of interest (e.g. fluorescence). The binary separation is determined by the setting of a threshold or "gate" (also sometimes called a trigger.) While the value of this "gate" or trigger is adjustable (i.e. quantitatively), the sorting process is still binary based on the threshold setting.
Furthermore, the rate of cell separation is relatively slow due to the fact that FACS sorters operate by examining a single cell at a time. Generally, a FACS sorter can provide a cell sorting rate of 10.sup.3 cells/second. Higher cell sorting rates are possible, however, these higher sorting rates may damage some cells. Still further, FACS sorters are relatively expensive and thus, most laboratory facilities are equipped with a limited number sorters.
Another technique employing cell tagging as a basis for separation is known as High Gradient Magnetic Separation (hereinafter HGMS). The concept of sorting materials based on their magnetic responsiveness was first introduced in the industrial and mining arts. These methods relied on the intrinsic magnetic properties of the sorted material (generally, iron (i.e. magnetic) from non-iron parts (i.e. non-magnetic)) as a basis of operation. See U.S. Pat. No. 2,056,426 issued to Frantz, "Magnetic Separation Method and Means," in 1936.
More particularly, in HGMS, a heterogeneous cell population, which includes a cell sub-population having magnetic cell tags, is passed through a magnetic field. As the heterogeneous cell population passes through the magnetic field, the cell sub-population labeled with the magnetic cell tags, becomes magnetically responsive to the applied magnetic field. That is, the cell sub-population including the magnetic tags will be subjected to a magnetic force which will cause the cells to be either attracted to (in the typical case), or repelled from, the magnetic field's source. Typically, the cell sub-population having the magnetic tags is attracted to the source of the magnetic field and collected by adhering to the magnetic source itself, or adhering to a cell collector device situated near the magnetic source. Therefore, the primary mode of the HGMS is also binary in nature, that is, it determines whether a cell has the magnetic tags or not.
The HGMS system, however, also has several drawbacks. Firstly, the cell sub-population of interest can be physically damaged during the HGMS process because of their forced magnetic massing at the collector device. Secondly, because the HGMS process sorts cells on the same fundamental principle as the FACS system, the HGMS method is also binary in nature. That is, both the FACS and HGMS systems separate cells based on the presence or absence of a parameter of interest (i.e. fluorescence and magnetic responsiveness, respectively).
While binary separation techniques are important, there is a desire in the biological sciences to fractionate cells in an "analog" fashion, that is, to sort cells based on the amount of tags bound to the cell. As mentioned above, binary cell sorting methods sort cells based on the presence or absence of a parameter of interest (i.e. magnetic responsiveness). Accordingly, it is an object of the present invention to provide a system and method for sorting cells based on the amount of tags bound to the cell.